Kinetics and mechanism of oxidation of dimethyl sulphoxide by sodium bromate–sodium bisulphite reagent in aqueous medium

Rates of oxidation of dimethyl sulphoxide (DMSO) by HOBr produced in situ from sodium bromate-sodium bisulphite reagent have been studied iodometrically in aqueous medium. The order in [DMSO] is one when [DMSO] < 0,01 mol dm–3, fractional when [DMSO] is between 0,01 and 0,5 mol dm–3 and zero when (DMSO) > 0,5 mol dm–3. Different rate laws are operative under these three conditions though HOBr is the effective oxidizing species in all the cases. A mechanism involving an intermediate four-membered cyclic transition state between DMSO and HOBr (formation constant K), which decomposes in a slow step with a rate constant (k) has been proposed. Thermodynamic parameters for the adduct formation step and activation parameters for the first-order decomposition of the adduct step have been evaluated in the temperature range 308–323 K. Activation parameters have also been determined while the orders in [DMSO] are unity and zero. The reaction product has been identified as dimethyl sulphone (DMSO2).